Abstract The overall goal of this proposal is to elucidate the thiol redox mechanisms that increase xenobiotic toxicity, alter neurite outgrowth, and enhance neurodegeneration. The thiol redox proteome is the adaptive interface between the genome and exposome, providing a means to sense, avoid, and defend against oxidants and other toxicants. Disruption of cellular thiol redox systems, e.g. thiol redox proteome, is a key feature of oxidative stress, contributing to age-related diseases, including neurodegeneration. Enhanced reactive oxygen species (ROS) production in a variety of conditions is linked to mitochondrial dysfunction. Thus, genetic factors or disease processes that cause increased basal levels of stress may result in increased susceptibility of certain populations to environmental exposures. Our preliminary data indicate that individuals with Down syndrome (DS) may be sensitive to the toxic effects of xenobiotics due to their enhanced basal levels of stress. DS is the most common genetic form intellectual disability and the cognitive phenotype can be highly variable. This variability cannot be completely explained by genetics. Additionally, due to a triplication of the amyloid precursor protein gene (APP), all DS patients develop Alzheimer's-like pathology. Based upon preliminary data and published reports, it is hypothesized that environmental exposures contribute to cognitive phenotype variability via disrupted thiol redox signaling and control due to enhanced basal levels of cellular stress and mitochondrial dysfunction. Because ER and oxidative stress are fundamental mechanisms of neurodegeneration, this proposal will investigate the role of redox signaling in the effects of MB on stem cells derived from DS patients, how these exposures affect neurite outgrowth, and how cognitive function is altered in a transgenic mouse model of DS. In Specific Aim 1, we will elucidate the mechanisms of enhanced MB toxicity in DS. Specifically, we will study the roles that oxidative stress, ER stress and mitochondrial dysfunction play in MB-mediated toxicity in DS. In Specific Aim 2, we will utilize iPS cell-derived neural progenitor cells and mature neurons from DS patients and euploid controls to evaluate disease- and toxicant- mediated changes in neurite outgrowth using high-content imaging techniques. Interventions will also be employed to investigate the impact of oxidative stress and ER stress on neurite outgrowth. Alterations in neuronal thiol redox proteome will also be determined using isotope-coded affinity tag (ICAT) redox proteomics. Lastly, in Specific Aim 3 an in vivo mouse model of DS, Dp(16)1Yey/+, will be used to determine the influence of DS and MB exposure on cognitive function. This aim will also characterize MB-mediated neurodegeneration of the hippocampus, and elucidates redox sensitive pathways altered in the hippocampus that impair learning and memory via ICAT redox proteomics. Successful completion of these aims will provide a mechanistic understanding of the role of ER stress, cellular redox status, and the thiol redox proteome in cognition and neurodegeneration.